How can i modify an already rigged model?
There's nothing wrong in the idea of modifying a character which is already rigged, as long as you know what you are doing:

The character you've downloaded has 134 bones, some deforming, some used as machinary, some constrained and/or related to other bones, some used with drivers, to perform deformations using a lot of shape-keys, expecially in the face (as usual).

Maybe you should start from a simplier character, for practicing, until you reach a good comprehension of what is going on.

Anyway, all deforming bones are related to vertexes, so you can freely edit their materials and textures, without affecting the rig.

Viceversa, if you extrude a vertex from another, it will copy the properties of the previous active one: if the new vertex stays in a similar position, very often there are no problems. A lot of problems generally can come out when shapeKeys are involved (all the face, in your case), in which case the new vertexes can seem to move in weird ways: any vertex modification in this zone probably will need a lot of adjustements in every Shape-Key in which it is involved.

Selecting the rig and entering edit mode you can add new bones, and parent them to the rest of the rig; you will have to create a new vertex group in the character's mesh, with the same name as the new bone, and assign a weight to the vertexes you want to move, for having it deforming the mesh.

Consider also that you can parent more than one mesh to the same rig, even if this solution  is not the best, often.

Search for Nathan Vegdahl tutorials serie about advanced rigging, he's the best!